(condition,extra) =
  case diagonal of
    NE | p2 == go N p1 -> (  True , go' N p2' )
    NE | p2 == go E p1 -> (  True , go' E p2' )
    SE | p2 == go S p1 -> (  True , go' S p2' )
    SE | p2 == go E p1 -> (  True , go' E p2' )
    SW | p2 == go S p1 -> (  True , go' S p2' )
    SW | p2 == go W p1 -> (  True , go' W p2' )
    NW | p2 == go N p1 -> (  True , go' N p2' )
    NW | p2 == go W p1 -> (  True , go' W p2' )
    _                  -> ( False , undefined )